There is a great need in industries such as the semiconductor industry for sensitive metrology equipment that can provide high resolution and non-contact evaluation capabilities, such as for product silicon wafers as those wafers pass through the implantation and annealing fabrication stages. In recent years a number of products have been developed for the nondestructive evaluation of semiconductor materials. One such product has been successfully marketed by the assignee herein under the trademark Therma-Probe (TP). This system incorporates technology described in the following U.S. Pat. Nos. 4,634,290; 4,636,088; 4,854,710; 5,074,669; and 5,978,074. These patents are each hereby incorporated herein by reference.
In one basic device described in these patents, an intensity modulated pump laser is focused on a sample surface for periodically exciting the sample. In the case of a semiconductor, thermal and carrier plasma waves are generated close to the sample surface that spread out from the pump beam spot inside the sample. These waves reflect and scatter off various features and interact with various regions within the sample in a way that alters the flow of heat and/or carrier plasma from the pump beam spot.
The presence of the thermal and carrier plasma waves affects the reflectivity R at the surface of the sample. Features and regions below the sample surface, such as an implanted region or ultra-shallow junction, can alter the propagation of the thermal and carrier plasma waves, thereby changing the optical reflective pattern at the surface. By monitoring the changes in R of the sample at the surface, information about characteristics below the surface can be obtained.
In the basic device, a second laser having a wavelength different from that of the pump laser is provided for generating a probe beam of radiation. This probe beam is focused collinearly with the pump beam and reflects off the sample surface. A photodetector is provided for monitoring the power of reflected probe beam. This photodetector generates an output signal which is proportional to the reflected power of the probe beam and is therefore indicative of the varying reflectivity. A lock-in detector is used to measure both the in-phase (I) and quadrature (Q) components of the signal. The two channels of the output signal, namely the amplitude (A2=I2+Q2) and phase (Θ=tan−1(I/Q)) channels, are conventionally referred to as the Modulated Optical Reflectivity (MOR) or Thermal Wave (TW) signal amplitude and phase, respectively.
In the MOR system described in these patents, pump and probe beams are used that each operate at a single wavelength. Characterization of a semiconductor sample is therefore based on a single-point correlation of experimentally obtained TW parameters (amplitude and/or phase) with the properties of interest. Due to the variety of thermal, optical, and electronic characteristics of a semiconductor that may change during the technological process, the ability of this single-point correlation to provide accurate information about sample properties is limited. This limited ability prevents a theoretical model from being applied to accurately and quantitatively describe various physical processes behind the TW signal.
Additional efforts to increase the measurement capabilities of these MOR systems included varying the distance between the pump and probe beam spots; varying the modulation frequency of the pump source; and combining the TW data with other measured data such as from photothermal radiometry (PTR), spectroscopic reflectometry, and/or ellipsometry. Such efforts are described, for example, in U.S. Patent Application Publication No. 2003/0150993, filed Dec. 10, 2002, and Application Publication No. 2003/0234932, filed May 16, 2003, as well as U.S. Pat. No. 6,532,070, each of which is hereby incorporated herein by reference. Many such “combined” systems, however, require separate measurement systems. Further, many existing systems are based on single-wavelength TW data, such that varying the modulation frequency and/or pump-probe beam offset in most cases results in featureless TW signal dependencies that are hard to use for quantitative analysis and comparison (fitting) to the theoretical model.
Other attempts to improve MOR system performance have each included an application-specific selection (or selections) of the optimal probe beam wavelength in order to increase photo-thermal signal amplitude. Examples of these efforts can be found in the following published articles: J. A. Batista et al., Anal. Sci. s73 (2001); G. Tessier et al., Appl. Phys. Lett. 78, 2267 (2001); and G. Tessier et al., Rev. Sci. Instrum. 74, 495 (2003). Each of these papers is hereby incorporated herein by reference. The approaches proposed in each of these publications still do not allow for a quantitative comparison of the experimental and theoretical dependencies. For example, in the Batista et al. paper, single-wavelength lasers are used to probe the thermal wave field at selected wavelengths, resulting in a set of experimental data that cannot be used for quantitative analysis. In the Tessier et al. publications, thermo-reflectance spectra are obtained without use of a pump beam by electrically heating the specimen.